This invention relates generally to the field of athermalization, and in particular to the athermalization of fiber-optic communications systems. More specifically, the invention relates to the athermalization of a wavelength router.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.
In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronicsxe2x80x94typically an electronic SONET/SDH system. However SONET/SDH systems are designed to process only a single optical channel. Multi-wavelength systems would require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology.
The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called xe2x80x9cwavelength routing networksxe2x80x9d or xe2x80x9coptical transport networksxe2x80x9d (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. New types of photonic network elements operating at the wavelength level are required to perform the cross-connect, ADM and other network switching functions. Two of the primary functions are optical add-drop multiplexers (OADM) and wavelength-selective cross-connects (WSXC).
In order to perform wavelength routing functions optically today, the light stream must first be de-multiplexed or filtered into its many individual wavelengths, each on an individual optical fiber. Then each individual wavelength must be directed toward its target fiber using a large array of optical switches commonly called as optical cross-connect (OXC). Finally, all of the wavelengths must be re-multiplexed before continuing on through the destination fiber. This compound process is complex, very expensive, decreases system reliability and complicates system management. The OXC in particular is a technical challenge. A typical 40-80 channel DWDM system will require thousands of switches to fully cross-connect all the wavelengths. Opto-mechanical switches, which offer acceptable optical specifications are too big, expensive and unreliable for widespread deployment. New integrated solid-state technologies based on new materials are being researched, but are still far from commercial application.
Exemplary wavelength routers are described in co-pending U.S. patent application Ser. No. 09/422,061, filed Nov. 16, 1999, pending, the complete disclosure of which is herein incorporated by reference. Such wavelength routers allow for flexible and effective routing of spectral bands between an input port and a set of output ports and vice versa.
One factor to consider when designing wavelength routers is the environment in which the routers are to be used. For example, wavelength routers may possibly experience temperature changes of up to about 55xc2x0 C. As a result, some of the components of the router may change in size, thereby affecting the operation of the router. For instance, such temperature changes may affect the focal length of the router and/or cause signals to drift off targets within the router.
Hence, this invention is related to techniques for athermalizing wavelength routers. In this way, the wavelength routes may successfully be used in a wide variety of environments.
The invention provides for the athermalization of wavelength routers so that the wavelength routers may acceptably operate within environments where the temperature varies. In one embodiment, a wavelength router comprises a base, a fiber optic input and output (FIO) component, a micro routing array (MRA) having a plurality of reflectors, and a MRA/FIO mount that mounts the MRA and the FIO component to the base. The router further comprises a grating and a grating mount that mounts the grating to the base at a location spaced apart from the FIO component and the MRA. At least one lens is positioned between the grating and both the MRA and the FIO component through which signals are transmitted. The base, the MRA/FIO mount and the grating mount are constructed of materials that expand and contract in a coordinated fashion when subjected to temperature changes such that a generally constant distance is maintained between the lens and the MRA/FIO mount. In this way, the focal length of the router remains essentially unchanged as the temperature of the router is changed. Further, the materials used in constructing the routers are configured to orient the grating relative to the routing array during temperature changes such that signals reflected from the grating are pointed toward the routing array.
To accomplish such features, the base may be constructed from a material such as Invar 36. The MRA/FIO mount may be constructed of a material such as titanium. Further, the grating mount may be constructed of a material such as titanium.
To maintain the generally constant focal length, the base may be provided with a coefficient of thermal expansion, CTEBP, that is less than the coefficient of thermal expansion of the MRA/FIO mount, CTEM. Further, if the MRA/FIO mount and the lens are spaced apart along a Z axis of the base, CTEBP and CTEM may be selected such that O=LMxc2x7(CTEBP+CTEM+FLxc2x7CTEBP), where LM is the length of the MRA/FIO mount along the Z axis, and FL is the focal length.
In another aspect, proper pointing may be accomplished by providing the grating mount with a face and a pair of legs. Further, one of the legs has a length that is longer than the other leg so that the face will reorient itself as the two legs expand and contract differently.
The invention further provides a method for routing signals using such a router. According to the method, a first signal is passed from an input of the FIO component, through the lens and onto the grating. The signal is reflected back through the lens and to the FIO component. The temperature of the router is subsequently changed and another signal is routed in a manner similar to the first signal. However, at the second temperature the base, the MRA/FIO mount and the grating mount have lengths that are different than when at the first temperature. Even so, the router is constructed such that the focal length and the routing angle remain generally constant at both temperatures.
The focal length and the point angle are maintained generally constant by constructing the base, MRA/FIO mount and grating mount of materials that are configured to expand or contract in a coordinated manner as the temperature changes. In this way, the focal length and point angle may be kept generally constant for temperatures ranging from about 0xc2x0 C. to about 55xc2x0 C.